Wiltshire Police said that, due to recent events in the city and concerns the pair had been exposed to an unknown substance, a “highly precautionary approach” had been taken by emergency services.

The force said in a statement: “Both were taken to Salisbury District Hospital and were clinically assessed. We can now confirm that there is nothing to suggest that Novichok is the substance. Both people remain in hospital under observation.

“The major incident status has now been stood down.”

Investigators were trying to establish if the pair had been victims of a crime and the restaurant remained cordoned off on Monday morning.

Police were called to the Italian restaurant on the corner of High Street and Crane Street by the South Western Ambulance Service at around 6.45pm.

A source briefed by the emergency services told the Press Association: “The ambulance crew at the site took the decision that the symptoms seemed consistent with Novichok poisoning which is why they called their colleagues in the police.”